Operational method for a cryogenic tunnel (1)

ABSTRACT

The invention is a method for operating a cryogenic tunnel through which products to be chilled or deep-frozen pass. This tunnel is equipped with means for injecting a cryogenic fluid and means for extracting the cold gases in the tunnel. The method includes obtaining an entry or an exit gas temperature from a gas temperature probe outside the tunnel in proximity to the tunnel entrance or exit. The method also includes obtaining an ambient temperature from an ambient temperature probe located outside the tunnel. The method also includes determining a first delta which is the difference between the ambient temperature and the gas temperature. And the method includes controlling the extraction rate of the extraction means by feedback as a function of the the temperature difference between the first delta and a set point, in order to restore the value of the first delta to the setpoint value if necessary.

BACKGROUND

The present invention relates to a method and a device for operating acryogenic tunnel, which tunnel is of the type through which products tobe chilled or deep-frozen pass and is equipped with means for injectinga cryogenic fluid as well as means for extracting the cold gasesresulting from the vaporization of the fluid in the tunnel at a variablerate.

A cryogenic tunnel is an open system through which products pass, whichare intended to be chilled or deep-frozen by injecting generally liquidnitrogen or some other cryogenic fluid which needs to be removed fromthe system in the form of a gas after vaporization.

The tunnel has an opening through which the products can enter and anopening through which the products can leave.

The cryogenic liquid enters the tunnel through one or more pipes.

One or more additional openings are generally dedicated to extractingthe cold gases resulting from the vaporization of the fluid in thetunnel, which therefore entails pumping out the gases containing a largeproportion of nitrogen and discharging them to the externalsurroundings.

In ideal operation, the gas flows should be balanced as follows:

-   -   Extraction rate=flow rate of nitrogen gas generated by the        liquid nitrogen injection.    -   Product exit side: air intake rate zero, and gas release rate        also zero.    -   Product entry side: ditto i.e. air intake rate zero and gas        release rate both zero.

It is virtually impossible to obtain such ideal operation in practiceand, in particular, it is very difficult to control the following twoaspects in a consistent way:

-   -   Matching the extraction rate to the volume of nitrogen gas        generated: the quantity of nitrogen injected into the tunnel is        variable in practice, and it may therefore be difficult to make        the extraction keep pace with the requirements.    -   Balancing the gases between the entry and the exit of the        tunnel: a tunnel may have a slightly negative pressure on the        product exit side and a slightly positive pressure on the        product entry side if the extraction rate is matched correctly,        even though the situation may become reversed a moment later.

Various approaches have therefore been proposed in order to providesolutions to the problems listed above.

Most frequently, “over-extraction” is performed in order to preventreleases of gas (and therefore leaks of nitrogen into the productionpremises).

This typically involves extraction at a fixed rate, which is calculatedwith a large safety margin relative to the maximum requirements of thetunnel, with suction hoods being located at the entry and exit of thetunnel.

The following characteristics are observed in such a case:

-   -   the extraction rate is much more than the flow rate of nitrogen        gas generated by the liquid nitrogen injection.    -   Product exit side: the air intake rate is much more than 0,        while the gas release rate is almost zero.    -   Product entry side: ditto i.e. an air intake rate much more than        0, while the gas release rate is almost zero.

It will therefore be understood that the advantage of this technicalsolution is that the risk of anoxia (cumulative nitrogen leaks in theproduction premises leading to a reduced level of oxygen in the room) islow when the tunnel is started up, but its drawback is associated withthe large intakes of air which cause moisture to enter the tunnel. Onthe inside, the equipment therefore ices up rapidly and loses itsefficiency. This intake of air also leads to an over-consumption ofnitrogen.

It should be noted that these intakes of air also cause moisture toenter the extraction lines, and therefore the creation of ice in them.After several hours of operation, this ice may obstruct the extractionlines and lead to nitrogen leaks from the tunnel due to lack ofextraction (whence a risk of anoxia).

Another solution encountered quite frequently in the industry, in orderto limit the intakes of air and releases of gas, is one according towhich the extraction is only slightly more than required (“slightover-extraction”). This is often the best compromise which can be foundin the state of the art.

According to this solution, extraction is performed at a fixed ratewhich is calculated to be just above the maximum requirements of thetunnel, or alternatively variable-rate extraction indexed to the degreeof opening of cock letting liquid nitrogen into the tunnel.

The following characteristics are observed in such a case:

-   -   the extraction rate is more than the flow rate of nitrogen gas        generated by the liquid nitrogen injection.    -   product exit side: the air intake rate is slightly positive,        with greater or lesser variations according to the operating        phases of the tunnel, while the gas release rate is slightly        negative on average, here again with greater or lesser        variations according to the operating phases of the tunnel.    -   product entry side: here again the air intake rate is slightly        positive on average, while the gas release rate is slightly        negative on average.

It can therefore be seen that the balance between the exit and the entryof the tunnel may vary over time, and that an observable situation inwhich gases are released from the entry of the tunnel and air is takenin at the exit of the tunnel may change to a situation in which air istaken in at the entry of the tunnel and gases are released from the exitof the tunnel.

It will therefore be understood that the advantage of this “slightover-extraction” solution is that the risk of anoxia is quite low whenthe tunnel is started up, while its major drawback, just as in the caseof over-extraction, is associated with the fact that the intake of aircauses icing of the equipment and of the extraction lines, and anover-consumption of nitrogen. The air intake rate, however, is low andthe technical drawbacks listed above are then more or less limiteddepending on the case.

A last approach may also be mentioned, although it is almost neveremployed in practice, which involves using reduced pumping in order tolimit the intakes of air (“under-extraction”).

The following characteristics are observed in such a case:

-   -   an extraction rate less than the flow rate of nitrogen gas        generated by the liquid nitrogen injection.    -   product exit side: an almost zero air intake rate, while the gas        release rate is positive.    -   product entry side: again, an almost zero air intake rate with a        positive gas release rate.

The advantage of the situation is indeed that no air is taken in at theentry and exit of the tunnel. No ice is therefore deposited in theequipment and in the extraction lines, and there is no over-consumptionof nitrogen due to possible intakes of hot air.

But it is quite clearly dangerous to operate a tunnel under theseconditions. The leaks of nitrogen to the outside of the tunnel entail arisk of anoxia and therefore a situation which is dangerous for thepersonnel working nearby.

The above discussion therefore demonstrates the genuine need to be ableto provide a solution that offers a better compromise for this industry,making it possible to work closer to the ideal equilibrium. To that end:

the extraction rate should be matched to the volume of nitrogen gaswhich is generated. Since the quantity of nitrogen injected into thetunnel is variable, the extraction rate should also keep pace with therequirements as accurately as possible while allowing for the possiblelags between the injection of liquid nitrogen and the moment when itvaporizes.

-   -   concerning the balance of the gases between the entry and exit        of the tunnel: the system should make it possible to guide the        gases in order to prevent them being released from either the        entry or the exit of the tunnel.    -   all these checks are preferably automatic without any human        intervention other than fixing the initial settings.

With such balancing of the gases in the tunnel and an extraction whichis fully matched to the requirements, the tunnel would thus no longertake air in (either at the entry or at the exit) and could thereforeoperate for a longer time without de-icing and without losing itsefficiency. The extraction lines would no longer be obstructed, and theleaks of nitrogen would at the very least be significantly reduced oreven eliminated. This would overcome the risk of anoxia.

The approach of Document U.S. Pat. No. 5,878,582 may also be mentioned,which attempts to control a cryogenic chamber by comparing a temperaturevalue at the external entry of the tunnel with a setpoint, and byfeedback control of the extraction means of the chamber according to theresult of this comparison.

The Applicant has been able to show that although this technicalapproach offers certain improvements over the prior-art approachesmentioned above, it is still unsatisfactory quite simply because it doesnot take account of the ambient temperature in the premises where thecryogenic chamber is operating.

Specifically, the setpoint temperature should be close to the ambienttemperature in order to obtain good results according to this document,while always remaining lower than it. This is because if the setpointbecomes higher than the ambient temperature (since the ambienttemperature has fallen), then the system becomes inoperable because theextraction will accelerate endlessly but without ever being able toreach this setpoint temperature. It will be impossible to increase themeasured temperature above the temperature of the ambient air. In short,the system can be controlled easily according to this technique if theambient temperature in the premises is relatively stable (plus or minusone degree), but when the temperature of the premises varies (which isoften the case in food production premises) then this control techniquemay become inefficient or occasionally inoperable (setpoint temperaturebecoming higher than the ambient temperature).

In one aspect of the present invention a method for operating acryogenic tunnel through which products to be chilled or deep-frozenpass is provided. This tunnel is equipped with means for injecting acryogenic fluid as well as means for extracting, at a variable rate,some of the cold gases resulting from the vaporization of the fluid inthe tunnel. The method includes obtaining a gas temperature, whereinthis gas temperature comprises a value selected from the groupconsisting of the temperature of the gases in proximity to the entry tothe tunnel, and the temperature of the gases in proximity to the exit tothe tunnel, wherein this gas temperature is obtained from at least onegas temperature probe which is provided outside the tunnel, at alocation selected from the group consisting of proximity to the tunnelentrance, and proximity to the tunnel exit. The method also includesobtaining an ambient temperature, wherein this ambient temperature isobtained from at least one ambient temperature probe which is providedoutside the tunnel. The method also includes determining a first delta,wherein this first delta is the difference between the ambienttemperature and the gas temperature. The method also includes comparingthe value of the first delta with a first setpoint value. And the methodincludes controlling the extraction rate of the extraction means byfeedback as a function of the result of the comparison in step d), inorder to restore the value of the first delta to the setpoint value ifnecessary.

In this context, the invention relates to a method for operating acryogenic tunnel through which products to be chilled or deep-frozenpass, which tunnel is equipped with means for injecting a cryogenicfluid as well as means for extracting, at a variable rate, some or allof the cold gases resulting from the vaporization of said fluid in thetunnel, characterized in that:

a) at least one temperature probe is provided outside the tunnel, inproximity to its entry and/or its exit, which is capable of providing avalue T_(entry/exit) of the temperature of the gases at the point whereit is located;

b) at least one temperature probe is provided outside the tunnel, whichis capable of providing a value T_(amb) of the ambient temperature ofthe premises where the tunnel is operating;

c) the difference T_(amb-entry/exit) between said ambient temperatureT_(amb) and said temperature T_(entry/exit) is determined, oralternatively the difference between the average of the ambienttemperatures which are provided by said ambient temperature probes andthe average of said temperatures T_(entry/exit) which are provided bysaid entry/exit temperature probes;

d) the value of the temperature difference provided by step c) iscompared with a predetermined setpoint value T⁰ _(amb-entry/exit);

e) the extraction rate of said extraction means is controlled byfeedback as a function of the result of the comparison in step d), inorder to restore the value of said temperature difference to saidsetpoint value T⁰ _(amb-entry/exit) if necessary.

The Applicant has therefore demonstrated the fundamental importance oftaking into account the ambient temperature of the premises where thetunnel is operating, in order to obtain high-quality operation. It canbe seen that the ambient temperature probe should preferably be arrangedat a position where the temperature is not influenced by the tunnel orby any other machine or ventilation system which may be present in thepremises in question.

The operating method according to the invention may furthermore adoptone or more of the following technical features:

-   -   regulation of the PID type is used in order to carry out said        feedback in step e).    -   one or more gas equilibration valves are provided inside the        tunnel, which is/are capable of directing the cold gases to the        entry or the exit of the tunnel and can be actuated        automatically from outside the tunnel.    -   in the case when said valves are present:

i) at least one temperature probe is provided outside the tunnel, inproximity to its exit, which is capable of providing a value T_(exit) ofthe temperature of the gases at the point where it is located, and atleast one temperature probe is provided outside the tunnel, in proximityto its entry, which is capable of providing a value T_(entry) of thetemperature of the gases at the point where it is located;

j) the difference T_(exit-entry) between said temperature T_(exit) andsaid temperature T_(entry) is determined, or the difference between theaverage of the temperatures T_(exit) which are provided by said exittemperature probes and the average of said temperatures T_(entry) whichare provided by said entry temperature probes;

k) the value of the temperature difference provided by step j) iscompared with a predetermined setpoint value T⁰ _(exit-entry);

l) the orientation of some or all of said equilibration valves iscontrolled by feedback as a function of the result of the comparison instep k), in order to direct some or all of the cold gases contained inthe tunnel so as to restore the value of said temperature difference tosaid setpoint value T⁰ _(exit-entry) if necessary.

-   -   regulation of the PID type is used in order to carry out said        feedback in step l).    -   said extraction means on which the feedback is carried out        comprise a single extraction line located inside the tunnel,        substantially above the region where the products enter.

The invention also relates to a device for operating a cryogenic tunnelthrough which products to be chilled or deep-frozen pass, which tunnelis equipped with means for injecting a cryogenic fluid as well as meansfor extracting, at a variable rate, some or all of the cold gasesresulting from the vaporization of said fluid in the tunnel, comprising:

a) at least one temperature probe located outside the tunnel, inproximity to its entry and/or its exit, which is capable of providing avalue T_(entry/exit) of the temperature of the gases at the point whereit is located;

b) at least one temperature probe located outside the tunnel, which iscapable of providing a value T_(amb) of the ambient temperature of thepremises where the tunnel is operating;

c) a data acquisition and processing unit capable of determining thedifference T_(amb-entry/exit) between said ambient temperature T_(amb)and said temperature T_(entry/exit), or alternatively the differencebetween the average of the ambient temperatures which are provided bysaid ambient temperature probes and the average of said temperaturesT_(entry/exit) which are provided by said entry/exit temperature probes,of comparing the value of the temperature difference provided by theprevious step with a predetermined setpoint value T⁰ _(amb-entry/exit),and of optionally controlling the extraction rate of said extractionmeans by feedback as a function of the result of the previouscomparison, in order to restore the value of said temperature differenceto said setpoint value T_(amb-entry/exit) if necessary.

The operating device according to the invention may furthermore adoptone or more of the following technical features:

-   -   the data acquisition and processing unit uses a regulator of the        PID type in order to carry out said feedback.    -   the device comprises one or more gas equilibration valves inside        the tunnel, which is/are capable of directing the cold gases to        the entry or the exit of the tunnel and can be actuated        automatically from outside the tunnel.    -   in the case when said valves are present, the device also        comprises:

i) at least one temperature probe located outside the tunnel, inproximity to its exit, which is capable of providing a value T_(exit) ofthe temperature of the gases at the point where it is located, and atleast one temperature probe located outside the tunnel, in proximity toits entry, which is capable of providing a value T_(entry) of thetemperature of the gases at the point where it is located;

j) a data acquisition and processing unit capable of determining thedifference T_(exit-entry) between said temperature T_(exit) and saidtemperature T_(entry), or the difference between the average of thetemperatures T_(exit) which are provided by said exit temperature probesand the average of said temperatures T_(entry) which are provided bysaid entry temperature probes, of comparing the value of the temperaturedifference provided by the previous step with a predetermined setpointvalue T⁰ _(exit-entry), and of optionally controlling the orientation ofsome or all of said equilibration valves by feedback as a function ofthe result of the comparison in step k), in order to direct some or allof the cold gases contained in the tunnel so as to restore the value ofsaid temperature difference to said setpoint value T⁰ _(exit-entry) ifnecessary.

-   -   said data acquisition and processing unit uses a regulator of        the PID type in order to carry out said feedback.    -   said extraction means on which the feedback is carried out        comprise a single extraction line located inside the tunnel,        substantially above the region where the products enter.

The invention also relates to a cryogenic tunnel which incorporates suchoperating means as described above.

BRIEF DESCRIPTION OF THE DRAWINGS

For a further understanding of the nature and objects for the presentinvention, reference should be made to the following detaileddescription, taken in conjunction with the accompanying drawings, inwhich like elements are given the same or analogous reference numbersand wherein:

FIG. 1 illustrates a stylized view of a prior-art tunnel in longitudinalsection;

FIG. 2 illustrates a stylized view in longitudinal section of a tunnelfor carrying out the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

In one aspect of the present invention a method for operating acryogenic tunnel through which products to be chilled or deep-frozenpass is provided. This tunnel is equipped with means for injecting acryogenic fluid as well as means for extracting, at a variable rate,some of the cold gases resulting from the vaporization of the fluid inthe tunnel. The method includes obtaining a gas temperature, whereinthis gas temperature comprises a value selected from the groupconsisting of the temperature of the gases in proximity to the entry tothe tunnel, and the temperature of the gases in proximity to the exit tothe tunnel, wherein this gas temperature is obtained from at least onegas temperature probe which is provided outside the tunnel, at alocation selected from the group consisting of proximity to the tunnelentrance, and proximity to the tunnel exit. The method also includesobtaining an ambient temperature, wherein this ambient temperature isobtained from at least one ambient temperature probe which is providedoutside the tunnel. The method also includes determining a first delta,wherein this first delta is the difference between the ambienttemperature and the gas temperature. The method also includes comparingthe value of the first delta with a first setpoint value. And the methodincludes controlling the extraction rate of the extraction means byfeedback as a function of the result of the comparison in step d), inorder to restore the value of the first delta to the setpoint value ifnecessary.

FIG. 1 illustrates the typical structure of a cryogenic tunnel 1 throughwhich products to be chilled or deep-frozen pass (product entry 7,processed-product exit 8), which tunnel is equipped with means 2 forinjecting a cryogenic fluid as well as one or more means 3 forextracting the cold gases resulting from the vaporization of said fluidin the tunnel. The presence of a series of fans 4 is furthermore shown.

The arrows 5 also represent the intakes of air into the tunnel (at theentry or exit) and the arrows 6 represent the releases of gas from thetunnel (also at the entry or exit).

The installation represented in FIG. 2 in turn makes it possible tocarry out the present invention. It will be noted that structuralelements that are the same as in FIG. 1 have the same reference (forexample the injection of cryogenic liquid 2, or the intakes of air 5into the tunnel or the releases of gas 6 from this tunnel).

In the embodiment which is represented, a temperature probe 21 isprovided outside the tunnel in proximity to its entry, which is capableof providing a value T_(entry) at the point where it is located, atemperature probe is provided outside the tunnel in proximity to itsexit, which is capable of providing a value T_(exit) of the temperatureof the gases at the point where it is located, and a temperature probe23 is provided outside the tunnel, which is capable of providing a valueT_(amb) of the ambient temperature of the premises where the tunnel isoperating.

The notion of “proximity” with respect to one or other of the probesaccording to the invention should be understood as meaning a reasonabledistance so that the delivered temperature value is representative ofthe air intake phenomena or cold-gas leakage phenomena, and, typically,an order of magnitude of from a few millimeters to a few tens ofmillimeters from the entry or exit door of the tunnel will therefore bevery suitable for carrying out the present invention.

As indicated in the figure, a data acquisition and processing unit 30 isalso provided (see the dashed and dot-and-dash arrows in the figure)which is capable:

-   -   of determining the difference T_(amb-entry/exit) between the        ambient temperature T_(amb) provided by the probe 23 and one or        other of the temperatures T_(entry/exit) provided by the probes        21 and 22, or their average;    -   of comparing the value of the temperature difference provided by        the previous step with a predetermined setpoint value T⁰        _(amb-entry/exit);    -   of controlling the extraction rate of the extraction means by        feedback as a function of the result of this comparison, in        order to restore the value of said temperature difference to        said setpoint value T⁰ _(amb-entry/exit) if necessary.

According to one of the embodiments of the invention, however, the unit30 is also capable:

-   -   of determining the difference T_(exit-entry) between the        temperature T_(exit) provided by the probe 22 and the        temperature T_(entry) provided by the probe 21;    -   of comparing the value of the temperature difference provided by        the previous step with a predetermined setpoint value        T_(exit-entry); and    -   of controlling the orientation of some or all of the        equilibration valves 20 by feedback as a function of the result        of this comparison, in order to direct some or all of the cold        gases contained in the tunnel so as to restore the value of said        temperature difference to the setpoint value T⁰ _(exit-entry) if        necessary.

Although it is possible to manage just the extraction 3 according to theinvention, it is clear that the combined use of both control modes(extraction means and valves) offers the best results.

The unit 30 determines the difference T_(exit-entry) between thetemperature T_(exit) (22) and the temperature T_(entry) (21), andcompares it with a predetermined setpoint value T⁰ _(exit-entry). If themovements of gas are taking place from the front to the rear in thetunnel, then air will be taken in at the entry of the tunnel, soT_(entry) will rise, and cold gases will also be released from the exitof the tunnel and T_(exit) will fall. Overall, the movement of gas fromthe front to the rear will lead to a reduction in T_(exit-entry).

A movement of gas from the rear to the front of the tunnel will likewiselead to an increase in T_(exit-entry).

Inside the tunnel, the gas equilibration valves 20 deviate theturbulence created by the fans and make it possible to direct the coldgases to the entry or exit of the tunnel, according to the requirements.

The invention therefore provides a means of controlling the movements ofgas in the tunnel (gas valves) and a means of measuring these movements(T_(exit-entry)). A regulating mechanism then makes it possible to adaptthe position of the gas valves continuously as a function ofT_(exit-entry) so as to obtain a stable situation without movement ofgas to the front or to the rear. A regulating system of the PID typecompares T_(exit-entry) with a setpoint and calculates the idealposition of the gas valves.

Temperature setpoints which, to a greater or lesser extent, are lowerthan the ambient temperature will preferably be used—whether for theentry or the exit—and in practice ones that are preferably close to 0°C.

It will be understood from reading the description given above thatthese control modes operate independently but, in combination, they makeit possible to obtain tunnel operation very close to the idealconditions.

Whatever the case, and without the following schematic explanation(which is purely intended to assist comprehension of the phenomena whichmay currently be encountered) implying any limitation of the presentinvention: when the two control modes are combined, there is a sort ofexchange of the “problem” between the entry and the exit of the tunnel(to deal with the intermediate “cold ball” lying between the entry andthe exit), with the valves being capable of sending this “cold ball” tothe entry while the extraction is in turn capable of removing some of itfrom the tunnel, when this proves necessary.

It will be understood that many additional changes in the details,materials, steps and arrangement of parts, which have been hereindescribed in order to explain the nature of the invention, may be madeby those skilled in the art within the principle and scope of theinvention as expressed in the appended claims. Thus, the presentinvention is not intended to be limited to the specific embodiments inthe examples given above.

1. A method for operating a cryogenic tunnel through which products tobe chilled or deep-frozen pass, which tunnel is equipped with means forinjecting a cryogenic fluid as well as means for extracting, at avariable rate, some of the cold gases resulting from the vaporization ofsaid fluid in the tunnel, wherein: a) obtaining a gas temperature,wherein said gas temperature comprises a value selected from the groupconsisting of the temperature of the gases in proximity to the entry tothe tunnel, and the temperature of the gases in proximity to the exit tothe tunnel, wherein said gas temperature is obtained from at least onegas temperature probe which is provided outside the tunnel, at alocation selected from the group consisting of proximity to the tunnelentrance, and proximity to the tunnel exit; b) obtaining an ambienttemperature, wherein said ambient temperature is obtained from at leastone ambient temperature probe which is provided outside the tunnel; c)determining a first delta, wherein said first delta is the differencebetween said ambient temperature and said gas temperature; d) comparingthe value of said first delta with a first setpoint value; and, e)controlling the extraction rate of said extraction means by feedback asa function of the result of the comparison in step d), in order torestore the value of said first delta to said setpoint value ifnecessary.
 2. The operating method of claim 1, wherein two or more saidambient temperature probes are provided, and an a average ambienttemperature is obtained therefrom.
 3. The operating method of claim 2,wherein an entrance gas temperature is obtained from said gastemperature probe which is located outside the tunnel at a location inproximity to the tunnel entrance, a exit gas temperature is obtainedfrom said gas temperature probe which is located outside the tunnel at alocation in proximity to the tunnel exit, and wherein an average gastemperature is the average temperature obtained therefrom.
 4. Theoperating method of claim 3, wherein said first delta is the differencebetween said average ambient temperature and said average gastemperature.
 5. The operating method of claim 1, wherein said feedbackis performed by a PID system.
 6. The operating method of claim 1,wherein one or more gas equilibration valves are provided inside thetunnel, which are capable of directing the cold gases to the entry orthe exit of the tunnel and wherein said equilibration valves can beactuated automatically from outside the tunnel.
 7. The operating methodof claim 6, further comprising: f) obtaining an entrance gastemperature, wherein said entrance gas temperature is obtained from atleast one gas temperature probe provided outside the tunnel, inproximity to its exit; g) obtaining an exit gas temperature, whereinsaid exit gas temperature is obtained from at least one gas temperatureprobe is provided outside the tunnel, in proximity to its entry; h)determining a second delta, wherein said second delta is the differencebetween said entrance gas temperature and said exit gas temperature, i)comparing the value of the said second delta with a second setpointvalue; and, j) controlling the orientation of some or all of saidequilibration by feedback as a function of the result of the comparisonin step h), in order to direct some or all of the cold gases containedin the tunnel so as to restore the value of said temperature differenceto said second setpoint value if necessary.
 8. The operating method ofclaim 7, wherein two or more said exit temperature probes are provided,and an average exit temperature is obtained therefrom.
 9. The operatingmethod of claim 8, wherein two or more said entrance probes are provide,and an average entrance temperature is obtained therefrom.
 10. Theoperating method of claim 9, wherein said second delta is the differencebetween said average exit temperature and said average entrancetemperature.
 11. The operating method of claim 7, wherein said feedbackis performed by a PID system.
 12. The operating method of claim 1,wherein said extraction means on which the feedback is carried outcomprises a single extraction line located inside the tunnel,substantially above the region where the products enter.
 13. A devicefor operating a cryogenic tunnel through which products to be chilled ordeep-frozen pass, which tunnel is equipped with means for injecting acryogenic fluid as well as means for extracting, at a variable rate,some or all of the cold gases resulting from the vaporization of saidfluid in the tunnel, comprising: a) at least one gas temperature probelocated outside the tunnel, in proximity to it's a location selectedfrom the group consisting of the tunnel entry and the tunnel exit,wherein said gas temperature probe is capable of providing a gastemperature value, wherein said gas temperature comprises a valueselected from the group consisting of the temperature of the gases inproximity to the entry to the tunnel, and the temperature of the gasesin proximity to the exit to the tunnel; b) at least one ambienttemperature probe located outside the tunnel, which is capable ofproviding an ambient temperature value of the premises where the tunnelis operating; and c) a data acquisition and processing unit capable ofdetermining a first delta, wherein said first delta is the differencebetween said ambient temperature and said gas, of comparing the value ofthe first delta with a first setpoint value, and of controlling theextraction rate of said extraction means by feedback as a function ofthe result of said comparison, in order to restore the value of saidtemperature difference to said first setpoint value if necessary. 14.The operating device of claim 13, wherein two or more said ambienttemperature probes are provided, and an a average ambient temperature isobtained therefrom.
 15. The operating device of claim 14, wherein anentrance gas temperature is obtained from said gas temperature probewhich is located outside the tunnel at a location in proximity to thetunnel entrance, a exit gas temperature is obtained from said gastemperature probe which is located outside the tunnel at a location inproximity to the tunnel exit, and wherein an average gas temperature isthe average temperature obtained therefrom.
 16. The operating device ofclaim 15, wherein said first delta is the difference between saidaverage ambient temperature and said average gas temperature.
 17. Theoperating method of claim 13, wherein said feedback is performed by aPID system.
 18. The operating device as claimed in claim 13, whereinsaid operating device further comprises one or more gas equilibrationvalves inside the tunnel, which are capable of directing the cold gasesto the entry or the exit of the tunnel and can be actuated automaticallyfrom outside the tunnel.
 19. The operating device of claim 17, furthercomprising: a) at least one gas temperature probe located outside thetunnel, in proximity to its exit, which is capable of providing anentrance gas, and at least one gas temperature probe located outside thetunnel, in proximity to its entry, which is capable of providing an exitgas temperature; and b) a data acquisition and processing unit capableof determining a second delta, wherein said second delta is thedifference between said exit gas temperature and said entrancetemperature, of comparing the value of the second delta with a secondsetpoint value, and of controlling the orientation of some or all ofsaid equilibration valves by feedback as a function of the result of theprevious comparison, in order to direct some or all of the cold gasescontained in the tunnel so as to restore the value of said second deltato said second setpoint value if necessary.
 20. The operating method ofclaim 19, wherein two or more said exit temperature probes are provided,and an average exit temperature is obtained therefrom.
 21. The operatingmethod of claim 20, wherein two or more said entrance probes areprovide, and an average entrance temperature is obtained therefrom. 22.The operating method of claim 21, wherein said second delta is thedifference between said average exit temperature and said averageentrance temperature.
 23. The operating method of claim 19, wherein saidfeedback is performed by a PID system.
 24. The operating device asclaimed in claim 13, wherein said extraction means on which the feedbackis carried out comprise a single extraction line located inside thetunnel, substantially above the region where the products enter.
 25. Acryogenic tunnel of the type through which products to be chilled ordeep-frozen pass, which is equipped with means for injecting a cryogenicfluid as well as means for extracting, at a variable rate, some or allof the cold gases resulting from the vaporization of said fluid in thetunnel, wherein said cryogenic tunnel comprises an operating device asclaimed in claim 13.